Pre heat oven to 180°C (160°C fan-forced). Grease and line two 22cm cake tins.
In a large bowl, combine and sift together flour, sugar, cocoa, bicarb soda, baking powder and salt.
In another bowl, use a mixer to beat together the oil, eggs, buttermilk and vanilla for 1 minute. Add the dry ingredients to the wet ingredients and stir with a wooden spoon until combined. Pour in hot water and mix together. Batter should appear liquidy to achieve a moist cake.
Pour the batter evenly into the two pans and bake for approximately 25 minutes or when cake springs back when lightly touched in the centre.
Leave cakes to cool in pan for 15 minutes before turning them out onto a wire rack. Allow the cakes to cool completely before frosting.
To make the Chocolate Buttercream: Cream together the butter, cocoa, icing sugar and milk until light and fluffy. Add more milk depending on consistency.
Spread buttercream between the layers, on the top and sides of the double layer cake.